1. Field of the Invention
This invention relates to delay devices for use with non-electric signal transmission lines of the type used in blasting operations, and more specifically to electronically controlled delay devices.
2. Background and Related Art
Blasting operations normally involve sequentially-timed detonations of explosive charges placed within bore-holes drilled into, for example, a rock or an ore mass to be fragmented. Generally, one or more transmission lines are deployed from a central initiating point to send a signal to detonate the individual blasting charges located within the respective boreholes. These transmission lines may consist of one or more main trunklines connected to a plurality of "downlines" leading from the trunklines into the boreholes to transmit the initiating signal to a detonator cap, sometimes referred to as a blasting cap, which, upon detonation, detonates the main explosive charge within the borehole.
The timing of sequential detonations within each borehole must be closely controlled to achieve the desired fragmentation and movement of ore and/or rock and to reduce noise and vibration. The desired time intervals between detonations are usually measured in milliseconds to achieve the desired results. Generally, at least an eight millisecond delay is employed between adjacent boreholes but significantly longer delays, e.g., 25, 50, 250 or 500 milliseconds, are often used. The desired timing of sequential detonations may be at least in part obtained by the use of blasting caps and, in some cases, signal transmission caps, which provide preselected delay periods between the receipt of the incoming signal and detonation of the caps. The provision of selected delays within blasting and signal transmission caps is well illustrated in Spraggs et al U.S. Pat. No. 3,987,732.
In many cases it is also necessary to provide one or more delays in the transmission of signals along the transmission lines and this may be accomplished by utilizing signal delay units in the transmission lines. The transmission lines may comprise conventional combustible fuses, explosive detonating cords, non-destructive signal transmission tubes, or combinations thereof, sometimes with supplementary charges utilized to ensure transmission of the signal between transmission lines at connection points. A supplementary charge device is disclosed in U.S. Pat. No. 4,481,884.
Conventional detonating cords are noisy and have a tendency to throw debris and shrapnel from destroyed connectors and the like, which may result in cutting the transmission line ahead of the signal, thereby disrupting the desired blasting pattern. These disadvantages in a transmission line may be overcome by the utilization of known, non-destructive signal transmission lines. One type is commonly referred to as "shock tube" and is illustrated in Thureson et al U.S. Pat. No. 4,607,573. Other non-destructive transmission lines include low velocity signal transmission tubes as illustrated in Thureson et al U.S. Pat. No. 4,757,764. Shock tube and low velocity signal transmission tube ("LVST tube") generally comprise hollow, plastic tubing which is coated on its interior surface with a thin layer of a suitable explosive (shock tube) or deflagrating composition (LVST tube). Upon initiation of the explosive or deflagrating composition within such signal transmission tubes, a shock wave, flame front or other such impulse signal is transmitted through the tube. This impulse signal may be utilized to detonate signal transmitting and blasting caps in order to initiate timed detonation of the main charges.
The use of pyrotechnic delay devices in signal transmission lines is known in the art. For example, a pyrotechnic delay unit for a signal transmission tube is shown in U.S. Pat. No. 4,742,773, issued to Bartholomew et al, on May 10, 1988. This Patent calls for using, in a signal transmission tube, a delay assembly comprising a delay element which contains a shaped pyrotechnic delay composition having a preselected combustion time. As described beginning at column 3, line 49 of the Bartholomew Patent, signal transmission tubes are received in the opposite ends of the delay assembly and connected to opposite ends of the delay element. An incoming impulse signal from one of the transmission tubes connected to the assembly initiates the timed combustion of the delay element, starting at one end thereof. The combustion time of the delay element may range from nine milliseconds to ten seconds or longer, depending on the delay composition utilized (column 4, lines 11-15). When the combustion proceeds from one end to the other end of the delay element, the preselected delay period will have elapsed and the burning delay element ignites the other, outgoing signal transmission tube. Consequently, a selected delay in timing of transmission of the signal through the transmission tube connected by the delay unit is attained. The pyrotechnic delay assembly of the Bartholomew Patent employs transition and delay chemical compositions comprising various reactive chemical compounds, as explained beginning at column 4, line 38.
Pyrotechnic delays such as those utilized in the Bartholomew Patent exhibit inherent variations in combustion time and hence in the desired delay interval. Consequently, the actual delay period of a given delay unit varies within a range of deviation from the nominal delay period of the unit. These variations are caused by compositional and manufacturing tolerances which are unavoidable as a practical matter in mass-produced pyrotechnic delay units and result in deviations sometimes referred to as "time scatter" in the planned timing of the sequence of detonations, and consequent poor blasting results. If the planned time interval between sequential detonations is very short, the time scatter may approach or even exceed the planned intervals, thus resulting in out-of-sequence detonations. Further, if it is desired to change the time delay of a given assembly, that can be accomplished only by replacing pyrotechnic delay units of a given delay period with pyrotechnic delay units of a different delay period. One must therefore maintain an inventory of pyrotechnic delay units of different delay periods in order to have the necessary flexibility in designing blasting patterns.
Electric blasting systems, in which sequential blasting machines are electronically controlled to deliver closely timed electrical signals to electrically activated instantaneous blasting caps do not have such time scatter problems. However, the use of electrical means as the sole source for timing the detonations makes it impossible to apply all of the signal pulses to a large number of charges prior to the detonation of the first charge, and this poses the danger that one or more electrical connections will be broken by the earlier explosions before the signal is delivered to the remaining charges. Further, electrical blasting systems present electrocution hazards and the possibility of premature or unintended detonations caused by static electricity and induced currents caused by power lines, ground currents or other sources.